1. Field of the Invention
The present invention relates to an image forming apparatus, such as a laser printer, a copying machine, a facsimile equipment, which use electrophotographic recording method. More particularly, the invention relates to the in-line type image forming apparatus for forming color image with a plurality of image bearing bodies by superposing the image formed on each image bearing body one after another on one and the same recording material.
2. Related Background Art
There have been proposed various image forming apparatuses that utilize electrophotographic recording method for the formation of color image on a recording material. Some of them have already been in practical use.
The image forming apparatus of electrophotographic recording method has an advantage in that it can make its recording speed faster than those using other recording methods, such as the ink jet method that forms images by spraying ink droplets directly to a recording sheet or the silver photographic method that records images by exposing them on a photosensitive coloring material. With this advantage, the image forming apparatus of the kind is designed to be different from those using other methods in order to meet the need on the market for higher speed recording.
As the typical example of color image forming apparatus utilizing electrophotographic recording method, there is the one in which a rotational developing apparatus is incorporated, for example. This rotational developing apparatus is provided with four developing devices containing four color developers (toners) of yellow, magenta, cyan, and black, along the periphery of the rotational body, and structured to be able to develop image one after another by use of each toner on the photosensitive body which serves as an image bearing body.
The image forming apparatus that adopts this rotational developing apparatus develops the electrostatic latent image of each color, which is formed on the photosensitive body, by means of the developing device of color corresponding to the developing position that faces the photosensitive body, thus making the electrostatic latent image visible as the toner image, and then, each time when the toner image is obtained on the photosensitive body, such toner image is transferred onto a sheet type recording material, such as recording sheet. This process is repeated to superpose each toner image of different color on the recording sheet for the formation of a color image.
As another example of the image forming apparatus that uses the rotational developing apparatus, there is devised the one which obtains color image on a recording sheet in such a manner that an electrostatic latent image is formed on a photosensitive body, and such image is developed repeatedly by use of the developing devices of corresponding colors so as to form on the photosensitive body the toner image having plural colors by superposing toner image of each color, and that the toner image having plural colors thus superposed is transferred onto a recording sheet altogether.
On the other hand, there is the so-called in-line type image forming apparatus in which a plurality of photosensitive bodies are provided, and each electrostatic latent image on each photosensitive body is developed by use of each developing device so as to from toner image of each color on each photosensitive body separately, and then, the toner image of each color is transferred to and superposed on the recording sheet which is carried to the plural photosensitive bodies sequentially, thus obtaining an color image on the recording sheet.
Here, there is also an in-line type image forming apparatus that adopts intermediate transfer, in which the toner image of each color is not transferred to a recording sheet directly, but transferred from each photosensitive body to and superposed once on an intermediate transfer body, and after that, transferred to a recording sheet together in order to provide a color image for the recording sheet.
These color image forming apparatuses of electro-photographic recording method have advantages, and some of disadvantages as well, but from the viewpoint of meeting the need on the market for higher speed recording in recent years, those of in-line type are regarded as superior, and there are many products of this type which have already been in use practically.
FIG. 9 is a side view which shows one structural example of the color image forming apparatus that adopts in-line method, and represents schematically the principal inner structure thereof. This image forming apparatus is structured in an in-line color printer of four-drum multiple transfer type.
This in-line color printer is provided with an endless transfer belt 6 which serves as a recording material carrying member. The transfer belt 6 is tensioned on a driving roller 7, a driven roller 9, and a tension roller 10, and driven to rotate in the direction indicated by an arrow A. Along the transfer belt 6, four photosensitive drums (drum type electro-photographic sensitive bodies) 1a, 1b, 1c, and 1d, which serve as image bearing bodies are arranged in series, and with the photosensitive drums 1a, 1b, 1c, and 1d, and other image forming means, image forming stations PY, PM, PC and PK are structured to form each of toner images of yellow, magenta, cyan, and black, respectively.
Generally, it is important for an in-line apparatus of the kind to make color deviations (color registration displacements) smaller among superposed images of plural colors in order to obtain a color image in high quality. Therefore, the interval between adjacent image forming stations PY to PK themselves, that is, the interval between the adjacent photosensitive drums 1a to 1d themselves, should be made equal. Thus, highly precise arrangements are required for the photosensitive drums (1a to 1d). Also, in order to make it difficult to allow color deviations to occur, there is a need for establishing designated relations between each interval of photosensitive drums (1a to 1d), and each outer diameter of photosensitive drums (1a to 1d), and the outer diameter of driving roller 7, respectively, and also, there is a need for rotating each of the photosensitive drums (1a to 1d) at an equal angular speed, among some others.
Each image forming means of the image forming stations PY, PM, PC, and PK comprises photosensitive drums 1a, 1b, 1c, and 1d; chargers (charging rollers) 2a, 2b, 2c, and 2d, arranged around them; exposing apparatuses 3a, 3b, 3c, and 3d; developing devices 4a, 4b, 4c, and 4d; and drum cleaners 15a, 15b, 15c, and 15d. Image forming means of each color is structured substantially the same with the exception of the developing devices 4a, 4b, 4c, and 4d each of which contains yellow, magenta, cyan, and black toner, respectively.
To describe the image forming operation in four full colors, each of the photosensitive drums (1a to 1d) is rotated at first so that each surface thereof is charged uniformly by the charging rollers (2a to 2d), respectively. Then, the laser beams modulated in accordance with image data are irradiated from each of the exposing apparatuses (3a to 3d) to form desired electrostatic latent image on the surface of each photosensitive drum (1a to 1d) corresponding to each color. The latent image on each of the photosensitive drums (1a to 1d) is reversely developed on the developing position by each of the developing devices (4a to 4d) by use of toner, thus being visualized as toner image each using yellow, magenta, cyan, and black toner.
The toner image of each color formed on each of the photosensitive drums (1a to 1d) is transferred electrostatically to and superposed one after another on the recording sheet P, which is carried on the transfer belt 6 for conveyance, by use of each of the transfer rollers (8a to 8d) of transfer means at each of the transfer nipping portions to face each of the photosensitive drums (1a to 1d), respectively. The recording sheet P is supplied from sheet feeding means (not shown) onto the transfer belt 6 by way of carrying means and borne on the transfer belt 6.
The four-color toner image multiply transferred onto the recording sheet P in such a manner is carried to a fixing device (not shown) where toner of each color is fused to mix colors and fixed. Thus, a desired printed image is obtained on the recording sheet P in colors.
When a monochromatic image in black is formed, image forming means PY to PC, other than the one for black, are not driven, and the photosensitive drums 1a to 1c and the transfer belt 6 are allowed to part from each other by use of a mechanism (not shown). In this condition, the same image forming operation is executed with respect to the photosensitive drum 1d of black image forming means PK.
The residual toner each on the photosensitive drums (1a to 1d) after transfer is removed by each of the drum cleaners (15a to 15d) provided with a cleaning blade and others so as to be prepared for the next image forming process.
In this respect, although there have been proposed various developing methods conventionally, it may be possible to apply developing devices (4a to 4d) of contact method or those of non-contact method to the image forming apparatus shown in FIG. 9. Also, with respect to the toner that serves as developer, it is possible to apply the one-component toner which can be used as toner individually or use the two-component toner which is used by mixing it with magnetic carrier, irrespective of types thereof. Here, as one example, a contact developing method that uses non-magnetic one-component toner may be cited.
In the image forming apparatus shown in FIG. 9, the photosensitive drums (1a to 1d), the chargers (2a to 2d), the developing devices (4a to 4d), and the cleaners (15a to 15d), among each image forming means of the image forming stations Pa to Pd, are integrally formed as each of process cartridges (113a to 113d) as shown in FIG. 10, and then, structured to be detachably attachable to the main body of image forming apparatus.
This image formation apparatus is provided with means for detecting the toner reminders in the aforesaid process cartridges (113a to 113d) and notifying the user thereof accordingly. Then, when a toner of certain color in a cartridge is consumed and the life thereof has been reached, the user is required to replace that particular cartridge only. Then, the user can use the apparatus continuously without any maintenance that should be carried out by a service engineer as in the case of a copying machine or the like.
With such a cartridge mode as described above, it becomes possible to obtain images in good condition stably at all times without drawback during the life thereof. There is also an advantage that the user can replace cartridges with ease when the life span thereof is reached. Also, as compared with the case where no cartridge mode is adopted, there is no need for replacing the photosensitive drum, the charger, the developing device, or the toner container or the like, which constitutes image forming means, each individually, thus suppressing the frequency of maintenance to a considerable extent.
Meanwhile, among color image forming apparatuses, there is the one that performs the so-called density control, that is, to optimize control by detecting patch densities, in order to uniformalize the densities of output images by modifying the control value of the developing bias, charging bias, or the like in several steps at a designated timing for the formation of designated patches which serve as toner images for detection use.
In the apparatus shown in FIG. 9, patches are formed on each photosensitive drum and transferred to the transfer belt 6. In this way, patches are formed on the transfer belt 6. Then, with reflection density detecting means (not shown), the patch densities are detected so as to execute the density control.
With the aforesaid patch formation, jamming of recording sheet, or the like, toner may adhere to the transfer belt 6 that serves as carrying passage. The adhesion of such toner is removed by the belt cleaner 11, which is provided with a cleaning blade or the like, arranged for the transfer belt 6 at a designated timing.
The toner thus removed from the transfer belt 6 and collected into the interior of cleaner 11 may be contained in a waste toner box detachable arranged for the cleaner 11 by way of a carrying screw or the like (not shown).
However, when the toner adhering to the transfer belt 6 should be collected by use of the belt cleaner 11 as in the case of the image forming apparatus shown in FIG. 9, the user is required to replace the cleaner 11 or the aforesaid waste toner box periodically. Thus, it becomes troublesome in terms of maintenance. Also, a space is needed for the arrangement of these devices to create a problem that the apparatus becomes larger eventually.
Further, it becomes necessary to provide a sensor for detecting the incapability of the cleaner 11 or the waste toner box to contain toner any longer, and means for notifying the user thereof accordingly or the like as well. As a result, a problem is also encountered that the costs of apparatus are made higher inevitably.
As a way to solve these problems, it is conceivable to adopt a mode in which the cleaners of the transfer belt 6 are eliminated as in the in-line color printer shown in FIG. 11, and the toner that adheres to the transfer belt 6 is returned to each photosensitive dram and collected by use of the drum cleaner thereof.
For the in-line color printer shown in FIG. 11, the belt cleaner is not arranged for the transfer belt 6 tensioned around two rollers, a driving roller 7 and a driven roller 9 as described above. The waste toner box is not arranged, either. Therefore, the apparatus itself can be made smaller, and also, waste toner detecting means or the like can be eliminated, thus implementing the cost down accordingly.
With the formation of patches described above, the jamming of recording sheet, or the like, toner is caused to adhere to the transfer belt 6. However, such toner is collected from each of the photosensitive drums to the respective drum cleaners subsequent to having been electrostatically returned to each of the transfer rollers that face to be in contact with the photosensitive drums via the transfer belt 6 utilizing the potential difference with the photosensitive drums by applying designated positive and negative cleaning biases at a designated timing of no image formation, that is, the timing at which toner on the transfer belt 6 (patch image, for instance) is cleaned beginning with the yellow station.
Nevertheless, when each amount of collected toners is examined, it is found that there is a considerable difference depending on the cleaners 15a to 15d. For this example, the collected toner is concentrated on the cleaner 15a at the yellow station PY, that is, the first color image forming station where cleaning is executed at first. As a result, depending on the condition in which the user uses the image forming apparatus, the capacity of cleaner 15a becomes insufficient so that the collected toner overflows from the cleaner. Thus, there may be encountered a problem that the waste toner is subjected to the so-called condition of puncture. As a result, it becomes necessary to perform a partial maintenance of cartridge, which deteriorates the maintainability of the apparatus as a whole.
Particularly for the user who outputs mainly the low-level print images having a smaller amount of prints, the process cartridges 113a to 113d are used for a long time without replacing any one of them. There is encountered a condition that the cleaner 15a is punctured by waste toner eventually despite toner still remains unused sufficiently in the developing device 4a of the cartridge 113a. 
Therefore, before the user is notified of the cartridge having reached the life span, the process cartridge 113a becomes unusable, and toner remaining in the developing device 4a is discarded wastefully. Also, if waste toner spreads inside the apparatus due to the toner puncture of cleaner 15a, critical damage is given to the apparatus eventually.
Meanwhile, with respect to the electrophotographic type color image forming apparatus, market researches are conducted as to the user behavior of actual use thereof. As a result, it has been confirmed that the consumption of black toner is great in terms of the total quantity of toners used, although there are the pictorial full color images like photographs, the one-point business color images which are partially colored only on the portion where emphasis is needed, and monochromatic images, among some others.
Therefore, as shown in FIG. 10, if the process cartridges 113a to 113d are structured in the same way, and the amount of toner content of each color is all the same for the developing devices 4a to 4d, toner puncture tends to occur by the cleaner 15d of the black cartridge 113d. As a result, the replacement frequency becomes more for the cartridge 113d so as to inevitably deteriorate the maintainability of the apparatus.